Within the integrated circuit industry there is a continuing effort to increase integrated circuit speed as well as device density. One challenge that integrated circuit designers face with increasing circuit speeds and device densities is the increasingly significant propagation delays of circuit inputs and outputs due to the capacitive loading associated with off-chip circuit connections. At slower clock speeds, the capacitive loading on integrated circuit lines is generally not a significant factor. However, as integrated circuit design clock speeds continue to climb towards the gigahertz range and beyond, it is evident that one of the major bottlenecks for future integrated circuits, such as for example, but not limited to, microprocessors, off-chip caches, controllers, etc. will be the input/output bandwidth and/or round trip delay between and within chips.
Prior attempts to address the capacitive loading problems associated with increased integrated circuit speeds and device densities have resulted in the use of larger and more powerful integrated circuit input/output drivers on the chip. Undesirable consequences of utilizing larger input/output drivers include the facts that the larger input/output drivers generally consume more power and induce noise. Further, adding large amounts of on-die decoupling capacitance to suppress noise results in more heat which needs to be dissipated and a requirement for more valuable integrated circuit die area.
Other prior attempts to overcome traditional integrated circuit interconnection limitations included the use of optical interconnections. The prior attempts at optical interconnections between integrated circuits have generally involved or have been based on two typical approaches.
One approach is based on either using gallium arsenide (GaAs) laser diodes and modulating or switching the diodes electrically or by using GaAs built modulators that amplitude modulate a laser beam passing through the integrated circuit. The modulation is generally based on electroabsorption through strained multi-layer grown molecular beam epitaxy (MBE) films in GaAs integrated circuits. As can be appreciated to those skilled in the art, it is difficult and therefore impractical to integrate or combine GaAs with silicon based metal oxide semiconductor (MOS) technology.
The second typical prior art approach is based on using silicon based optical waveguides. These waveguides are generally built using silicon-on-insulator (SOI) based processing techniques. Prior SOI based modulators utilize silicon waveguide structures to switch light passing through the optical waveguide. The switching mechanism utilizes injection of carriers into the waveguide, similar to a bipolar based transistor. One consequence of this is slow speed, for example up to several hundred megahertz, and very high power consumption, for example, 10 mW or more for a single switch. In order to increase the modulation depth, one often tries to obtain a large interaction volume between the injected charge and the light beam. This is generally accomplished by making very long waveguides, for example, on order of thousands of microns, thereby increasing the interaction length through which the light beam travels. As can be appreciated to those skilled in the art, actual incorporation of SOI waveguides into existing multi-layer standard CMOS based processing is not straightforward. Thus, utilization of these waveguide structures becomes quite impractical when used for high speed input/output in large transistor count microprocessors.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for an apparatus and method for optically modulating light using MOS fabrication technologies.